Bimetallic starter switch for gas discharge tubes



06L 1963 H. J J. VAN BOORT ETAL 3,105,889

BIMETALLIC STARTER SWITCH FOR GAS DISCHARGE TUBES Filed June 25, 1958 2 heets'5heet l /8 1: a 2 VA V//A% 2 FIG I FIG 2 27 FIG.3 28

INVENTOR HENRICUS \LJ. VAN BOORT JOHANNES H. HILDERINK k. l 4 5M Oct. 1963 H. J. J. VAN BOORT ETAL 3,105,839

BIMETALLIC STARTER SWITCH FOR GAS DISCHARGE TUBES Filed June 25, 1958 2 Sheets-Sheet 2 INVENTOR HENRIGUS Jul. VAN BOORT JOHANNE S H. HILOERINK AGENT discharge occurs.

United States Patent 3,105,889 BIMETALLIC STARTER SWITCH FOR GAS DiSCI-IARGE TUBES Henricus Johannes Joseph van Boort and Johannes Hendrilrus Hilderinir, both of Eindhoven, Netherlands, assignors to North American Philips Company, Inc,

New Yorlr, N.Y., a corporation of Delaware Filed June 25, 1958, Ser- No. 744,476

Claims priority, application Netherlands Aug. 17, 1957 3 Claims. (Cl. 200-113.5)

It is known to promote the starting of electric gas discharge tubes by means of automatic switches which are connected in parallel with the discharge path of said tubes. When the discharge tube is rendered operative, the switch is or has been closed so that the discharge path of the discharge tube is bridged and a current flows through the closed switch. This current is used either to heat any thermionic electrodes of the discharge tube or to produce a voltage source in an inductance which is connected in series with the discharge tube; naturally the two effects can also be realized simultaneously.

Various switches are known for use in such arrangements. Use is frequently made of thermal switches, that is to say, switches which are operated by heating a heatsensitive component which in many cases consists of a bimetal strip. Naturally in the arrangements described it is generally desirable to employ a switch which during the operation of the discharge tube consumes no or substantially no energy. For this purpose, use is frequently made of a switch comprising a small glow dischargetube having an electrode consisting of a bimetal strip. The gas pressure in this glow discharge tube must be made so that the starting voltage of this tube at the most is equal to the voltage supplied to the switch before the main discharge tube has started but is larger than the operating voltage of this main discharge tube. The gas pressure of thecommercially available glow discharge tube is lower than atmospheric pressure. The low gas pressure requires the provision of a switch casing which is sealed in a vacuum-tight manner and in which the glow In addition, this low gas pressure does not contribute toeflicient operation of the switch with respect to the production of avoltage surge for starting the .maindischarge tube. V

In another switch which has been described for use in the said arrangements, two bimetal strips, which are each provided with a contact, are heatedby a heater comprising a resistance wire of special composition. This heater, which is electrically connected between the bimetal strips and consequently parallel to'the contacts of the switch, consists of a material which is known, for example, from the British patent specification 292,110 and which has the property that the heat developed by the passage of current varies with a power greater than two of the impressed voltage. The resistance (voltage divided by current) of this material depends upon the impressed voltage in a manner such that it is smaller at a higher voltage than at a lower voltage. The extent to which the resistance varies with variation of the impressed voltage depends upon the composition of the material, which usually consists of silicon carbide with an admixture of carbon or some other conductor.

When this switch is used in parallel with the discharge path of a gas-discharge tube, the voltage between the ends of the heater is considerably lower subsequent to the ignition oflthe gas-discharge tube than prior to this ignition, so that the resistance of the heater subsequent to this ignition is also considerably larger than prior to this ignition and consequently the energy consumption of the heater during the operation of the gas-discharge tube is small compared with the energy which this heater consumes during the starting period.

"ice.

The present invention relates to a switch of the lastmentioned kind, that is to say, to a switch haying a bimetal element capable of moving a contact, which bimetal element can be heated by a heating element which is connected in parallel with the contacts of the switch and is made from a material the resistance of which is smaller at a higher impressed voltage than at a lower impressed voltage. v

It is an object of the invention to provide an improved switch of this kind inter alia by increasing the reaction velocity of the switch. I

According to the invention, the heater is plate-shaped and engages the bimetal elements in heat-conducting contact therewith. Owing to this non-filamentary shape of the heater, which for example may have the form of a parallelopiped, it can engage the bimetal element through a large surface area which ensures a satisfactory heat transfer between the heater and the bimetal element and consequently a quick response of this element. It has been found that the suitable materials can be employed satisfactorily in this plate-shape. Preferably, the heater directly engages the bimetal element without the interposition of a current supply member, the heater itself serving as a current supply member.

Means can be used advantageously to press the heater and the part of the bimetal element engaged by this heater to one another, thereby assisting in the heat transfer.

When the switch is closed owing to the passage of cur rent through the heater, that is to say when the contacts of the switch come into contact with each other, the heater is short-circuited so that it no longer passes current. Owing to the cooling down of the bimetal elements, the switch opens after some time. In order to shorten the time during which the switch is closed,:preferably attention should be paid tothe weight of the heater: this is preferably made less than 0.5 gm. i

The duration of the closure of the switch can also be influenced very eifectively by providing the switch with a second bimetal element, which is capable of moving the second contact and disposed so that the two contacts of the switch move in the same direction when the bimetal elements are heated. In this arrangement, heat is conveyed from the heater to the second bimetal element by a route such that the heatingof this second bimetal element lags with respect to the heating of the first bimetal element. Thus on operation of the switch the first bimetal element will be heated more rapidly so that the switch is closed... The second bimetal element, which reaches the appropriate temperaturewith a certain time lag, will again open the switch. By controlling this lag of the heating of the second bimetal element, the time during which the switch remains closed can be adjusted within wide limits. The heat transfer from the heater to the bimetal element can be effected advantageously with the aid of a metal body which is in heat-conducting contact with the heater and with the second bimetal element. By a suitable choice of the dimensions and the material of this metal member, the said time lag can be influenced and hence the period of closure of the switch can be controlled.

The reaction speed of the switch can be further increased by arranging the heater to engage that side of the bimetal element which, on heating, expands to a higher switch contains a second bimetal element which carries the movable first contact and receivesthe heat from the heater through the first bimetal element, the bimetal ele- 3 ments being arranged so as to move the contact in opposite directions on temperature variation.

When the switch is operated, the first bimetal element will be heated more rapidly. Owing to the deformation of this element, the switch is closed. The second bimetal element, which only reaches the appropriate temperature with some time lag through the first bimetal element, will again open the switch. By controlling this lag of the heating of the second bimetal element, the time during which the switch is kept closed can be adjusted within Wide limits.

A very simple embodiment is obtained when the fiat side of the end of the second bimetal element nearer the heater is secured to the flat side of the end of the first bimetal element remote from the heater. In this arrangement, the reaction velocity of the switch can be increased if the flat sides of the bimetal elements which are secured to one another, are the sides of the bimetal elements which exhibit the greater expansion on heating. 7

If the new switch described above is connected in a manner known per se in parallel with the discharge path of a gas and/or vapour discharge tube, which may be provided with at least one hot electrode and is connected in series with an impedance which may entirely or in part be in inductance, with suitable matching of the switch to the dimensions of the gas discharge tube a stanting arrangement is obtained for this tube which operates efiiciently and, during normal operation of the discharge tube, consumes a slight amount only of power which is substantially negligible compared with the load of the discharge tube. The resistance of the heater may be made to depend upon the voltage in a manner such that the power consumed by the switch subsequent to the ignition of the discharge tube is less than 10% or even less than of the power which the switch consumes before the discharge tube has been started. The absolute value of the power consumed by the switch subsequent to the ignition of the discharge tube can advantageously be made less than 0.5 watt.

If required, the bimetal elements and the heater of the switch can be accommodated in a casing sealed in a vacuum-tight manner; the composition and the pressure of the atmosphere in the casing can be chosen so that the closure of the switch takes place under favourable conditions. It has been found that in many cases this possibility can be dispensed with since frequently the switch is opened very satisfactorily in air under atmospheric pressure.

In order that the invention may be readily carried out, some embodiments thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawings.

FIG. 1 is a sectional view of one embodiment of a switch according to the invention;

FIG. 2 is a sectional view of another embodiment of a switch according to the invention;

FIG. 3 is a sectional view of still another embodiment of a switch according to the invention;

FIG. 4 is a sectional view of another embodiment of a switch according to the invention; and

FIG. 5 is a sectional view of the heater assembly of the switch shown in FIG. 4.

FIG. 1 is a side view of a switch comprising a bimetal strip 1 which is secured in an insulating plate 2 and is provided with a contact 3 made, for example, of tungsten. A tungsten rod 4, the slightly curved end 5 of which forms the second contact of the switch, is also secured in the plate 2. The heater 6 in the form of a plate of length 6 mms., width 5 mms. and thickness 0.6 mm, is applied to the bimetal strip 1. This heater 6 engages the bimetal strip 1 by one of its flat sides and can be clamped to this strip 1 by means of two bands (which are not shown for the sake of clarity). The side of the heater plate remote from the bimetal strip is coated with a thin metal layer 7 which, by means of a wire 8, is electrically connected to the rod 4. Thus, the heater is connected in parallel with the contacts 3 and 5. The electrical connection of the switch can be effected with the aid of the ends of the strip 1 and the rod 4 projecting through the plate 2.

The heater 6 is made of a material which may consist substantially of silicon carbide and the resistance of which is dependent upon the voltage. When the applied voltage is 220 volts, the resistance of the heater is 12,000 ohms and at 110 volts 80,000 ohms.

The side of the bimetal strip more remote from the heater 6 is made of a metal the coefiicient of thermal expansion of which exceeds that of the metal from which the other side of the strip is made. Consequently, when the bimetal strip 1 is heated its free end moves to the right until the contact 3 engages the contact 5. When the bimetal element 1 cools down the switch is opened again.

The heater 6 may alternatively be secured to the lefthand side of the bimetal strip 1, that is to say, to the side having the higher coefiicient of expansion. This increases the reaction speed of the bimetal element.

This increase can also be ensured by arranging the side of the bimetal element nearer the rod 4 to have the higher coefficient of expansion, the bimetal strip 1 being provided with a bent wire 9, as is shown in FIG. 2. When the strip =1 is heated, its upper end moves to the left, so that the part 10 of the wire 9 engages the right hand side of the end 11 of the rod 4.

FIG. 3 shows a third embodiment. This figure also shows diagrammatically a possible connection of the switch to a gas-discharge tube which is started with the aid of this switch.

In this figure, there are secured in a base plate 2 made of insulating material two connecting members 12 and 13, the member 12 being connected to a bimetal strip 14. A heater 15 is interposed between the bimetal strip *14 and a metal strip 16 arranged on the base plate 2. This base plate carries a second bimetal strip -17 which is connected to the connecting member 13. Part of the bimetal strip 14, the heater '15 and part of the metal strip 16 are pressed against the base plate by a clamping plate .18 made of insulating material.

To the upper end of the bimetal strip 14 there is secured a U-shaped metal brace 19 the free end 20 of which acts as the work contact of the switch. To the upper end of the bimetal strip '17 there is secured a metal wire 21 arranged at right angles to theplane of the drawing and acting as the counter contact of the switch.

The connecting members 12 and 13 are connected through thermionic electrodes 22 and 23 of a gas-discharge tube 24, which may be a fluorescent lamp of 40 watts, to terminals 27 and 28 of a current supply source, for example, an alternating current mains of 50 or 60 cycles and 2-20 volts, with the interposition of a ballast impedance in the form of a choke and a main switch 26.

The connecting members 12 and 13 of the switch can be interconnected by means of an anti-interference capacitor 29. The switch can be enclosed by a cover 30 shown by broken lines which is slipped over the base plate 2.

When the main switch 26 is closed, substantially the entire voltage of 220 volts is set up across the heater '15 which consists of a plate of weight about mgs., of diameter 5.5 mms. and thickness 0.6 mm, which plate is made from a material the resistance of which is highly dependent upon the voltage. At a voltage of 220 volts, the heater has a resistance such that it consumes a power of about 10 watts. The heat which is as a result developed in the resistance, is transferred directly to the bimetal strip 14, which engages the heater 15 by the layer of higher thermal expansion. Theother bimetal strip '17 is heated through the metal strip 16. Thus, the heating of the bimetal strip 17 lags with respect to that of the bimetal strip 14. This time lag can be controlled by a suitable choice of the physical dimensions of the strip 16. Thus, the work contact 20 starts to move in the direction of the arrows before the counter-contact 21. In its movement to the left the work contact strikes the counter contact 21 which is still substantially stationary so that the resistance 15 is short-circuited and a large current passes through the thermionic electrodes 22 and 23 of the discharge tube 24.

Owing to the short-circuiting of the heater 15 no further heat is developed in this heater. The amount of heat stored in it is such that it would take a comparatively long time for the work contact 20 to return in the direction opposite to the direction of the arrows and to reak its contact with the counter-contact 21. However, since the counter-contact 21 after some time starts to move in the direction of the arrows, the contact between the contacts 20 and 21 is interrupted by the removal of the counter-contact 21. Owing to this termination of the short-circuiting of the heater .15 the thermionic electrodes 22 and 23 are no longer heated, a voltage surge being produced by means of the choke 25, which is capable of starting the tube 24. Across the started discharge tube and the heater 15 connected in parallel therewith, there is now set up only the operating voltage of the discharge tube 24, which generally is about one half of the supply voltage. At this voltage, the resistance of the heater 15 is many times that prior to the ignition of the tube, the power consumption now being about 0.2 watt.

By way of illustration it should be mentioned that in a practical embodiment the dimensions of the bimetal strips 14 and 17 were about 28 x 3 x 0.2 mms. and 23 X 3 x 0.2 mms. respectively, those of the tombac strip 16 about 9 X 6 x 0.25 mms. At room temperature the spacing between the contacts 20 and 21 was about 0.3 mm. The time clapsing between the closure of the main switch 26 and the closure of the thermal switch was about 1 second. The thermal switch remained closed for about 2' seconds. When the contact2l1 was held stationary, the switch remained closed for about 10 seconds.

If the discharge tube 24 should not be started on the first closure of the switch, the switch is again closed and opened. {It will be appreciated that the thermal switch can also be dimensioned for other supply voltages, for example, for a voltage of 110 volts. The switch can also be used to heat the thermionic electrodes of the dis charge tube 24 without a choke being connected in series with the tube. This may be the case if the ballast impedance is an incandescent lamp. Alternatively, the discharge tube 24 may not be provided with thermionic electrodes and may be started with the aid of the voltage surge produced by the choke only.

'In FIGS. 4 and 5, reference numeral 32 designates a base plate of insulating material to which are secured two connecting members 33 and 34. The member 34 is connected, through a stay wire 85, to an L-shaped bimetal strip '36. A heater 37 is interposed between the horizontal part of the bimetal strip 36 and a clamping band 38 (of. FIG. also). This band is welded to sides 39 and 40 of an L-shaped metal strip 41.

To the flat side having the higher coefiicient of thermal expansion or" the vertical end of the bimetal element 36, there is welded the flat side of higher coefficient of thermal expansion of a second, I-shaped bimetal strip 42. To the free end thereof there is welded, at right angles to the plane of the drawing, a metal wire 43 which acts as the moving first contact or Work contact of the switch. To the vertical free end of the L-shaped metal strip 41 there is welded a metal wire 44 extending parallel to the plane of the drawing which acts as the second contact or counter-contact of the switch. The first contact 43 is connected, through the bimetal strips 42 and 36 and the stay wire 35, to the connecting member 34 while the second contact 44 is connected to the connecting member 33 through the metal strip 41 and a stay wire 45. The heater is connected in parallel with the contacts 43 and 44 through the clamping band 38, the sides 39 and 40 and the metal strip 41 on the one hand and the bimetal strips 36 and 42 on the other hand. Between the horizontal parts of the first bimetal strip 36 and the metal strip 41 there is provided an insulating plate 46.

The connecting members 33 and 34 of the switch are connected, through thermionic electrodes 47 and 48 of a gas discharge tube 49, for example a fluorescent lamp of 40 watts, with the interposition of an inductance 50 and a main switch 51, to terminals 52 and 53 of a current supply source, for example, an alternating current mains of 50 or 60 cycles and 220 volts.

The connecting members 33 and 34 can be interconnected by a radio-interference suppressing capacitor (not shown). The switch can be covered by a cover which is moved over the base plate 32 and is likewise not shown.

When the main switch 51 is closed, substantially the entire voltage of 220 volts is set up across the heater 37 which consists of :a plate of weight about 50 mgs., of diameter about 5.5 mms. and thickness about 0.8 mm, which plate is made of a material known per se, for example, silicon carbide with a graphite addition, the resistance of which is highly dependent upon the applied voltage. At a voltage of 220 volts the heater has a resistance such that it consumes a power of about 15 watts. The heat which as a result isdeveloped in the resistance, is transferred directly to the bimetal strip 36 whichcngages the heater 37 by its layer of largest thermal explansion. Hence, the vertical part of the L-shaped first bimetal strip36 curves to the left and brings the moving first contact 43 into contact with the stationary second contact 44 through the second bimetal strip 42 which is still cold. As a result, the heater 37 is shortcircuited and a large current passes through the thermionic electrodes 47 and 48 of the discharge tube 49.

Owing to the short-circuiting of the heating element 37, heat development in this element ceases. The amount of heat stored in it is such that it would take a comparatively long time before the work contact 43, owing to the cooling of the first bimetal strip 36, returns to the rest position and separates from the counter contact 44. Since the heat supplied by the heater reaches the second bi metal strip with a certain time lag through the first bimetal strip 36, this second bimetal strip subsequently curves to the right. Consequently, the resultant interruption of the contact between the contacts is due to the lagging heating of the second bimetal strip 42. The period of closure of the switch depends upon the value of the heat resistance between the bime-tal strips 36 and 42. This resistance can be increased by reducing the contact area between the bimetal strips or by connecting an additional heat resistance, for example in the form of a metal band, between the bimetal strips.

Owing to the separation of the contacts 43 and 44, the thermionic electrodes 47 and 48 are no longer heated, a voltage surge capable of igniting the tube 49 being produced with the aid of the inductance 50.

Across the ignited discharge tube and the heater 37 connected in parallel therewith there is setup the operating voltage of the discharge tube which as a rule is about one half of the supply voltage. At this volt-age, the heater resistance is many times that prior to the ignition, power consumption being only about 0.2 watt.

By way of illustration it should be mentioned that in a practical embodiment the physical dimensions of the bimetal strips 36 and 42 were about 10 x 3 x 02mins. and 15 X 3 x 0.2 mms. respectively. The vertical part of the 'bi-metal strip 36 was about 4 mms. long and the binietal strips overlapped for about 2 mms. At room temperature the spacing between the contacts 43 and 44 was about 0.3 mm. The time interval between the closure of the main switch 51 and the closure of the contacts 43 and 44 was about 1 second. The contacts remained closed for about 2 seconds. When the second bimetal strip 42 was replaced by an ordinary metal strip of the same dimensions, the contacts remained in contact until the first bimetal strip 36 had sufiiciently cooled, that is to say, for about seconds.

Owing to the opposed movements of the bimetal strips, the switch described has the property of keeping the contact spacing substantially constant even with fluctuations of the ambient temperature, since the bimetal strips are influence-d simultaneously.

What we claim is:

1. An electrical switch for a starting-device for gasfilled electric discharge tubes comprising a pair of normally open contacts, a first bimetallic element adapted upon heating to move one of said contacts to close the pair, a second bimetallic element engaging the first bimetallic element, and a plate-shaped heating element in direct physical-contact with said first bimetallic element, said heating element being connected in parallel with said contacts whereby the closing of said contacts short-circuits said heating element, said heating element being constituted of a material the resistance of which decreases with increasing voltage, asid second bimetallic element being disposed to move said one contact in an opposite direction upon being heated by said first bimetallic element to open said pair of contacts.

2. An electrical switch for a starting-device for gasfilled electric discharge tubes comprising a pair of normally open contacts, a first bimetallic element having a relatively fiat portion and adapted upon heating to move one of said contacts to close the pair, a second bimetallic element having a relatively flat portion in direct physical contact with the flat portion of the first bimetallic element, and a plate-shaped heating element engaging said first bimetallic element, said heating element being connected in parallel with said contacts whereby the closing of said contacts short-circuits said heating element, said heating element being constituted of a material the resistance of which decreases with increasing voltage, said second bimetallic element being disposed to move said one contact in an opposite direction upon "being heated by said first bimetallic element to open said pair of contacts.

3. An electrical switch for a starting-device for gas filled electric discharge tubes comprising a pair of normally open contacts, a first bimetallic element having a relatively flat portion one side of which is constituted of a metal having a higher thermal coeificient of expansion than the other, said first bimetallic element being adapted upon heating to move one of said contacts to close the pair, a second bimetallic element having a relatively fiat portion one side of which is constituted of a metal having a higher thermal coeflicient of expansion than the other, said side of said second bimetallic element having the higher thermal coeflicient of expansion engaging the side of the first bimetallic element having a higher thermal coefficient of expansion whereby said contact is moved first in a direction to close the pair of contacts when the first bimetallic element is heated and to move the contact in an opposite direction when the second bimetallic element is heated to open the pair of contacts, a plate-shaped heating element in direct physical contact with said first bimetallic element, said heating element being connected in parallel with said contacts whereby the closing of said contacts short-circuits said heating element, said heating element being constituted of a material the resistance of which decreases with increasing volt-age.

References Cited in the file of this patent UNITED STATES PATENTS 1,960,408 Brach May 29, 1934 2,248,623- Hand July 8, 1941 2,274,399 Cook Feb. 24, 1942 2,306,223 McCarthy Dec. 15, 1942 2,336,504 Ruben Dec. 14, 1943 2,338,474 Wilson Jan. 4, 1944 2,372,148 Whiteside Mar. 20, 1945 2,412,883 Gasser Dec. 7, 1946 2,427,714 Peters Sept. 23, 1947 2,852,640 De Lancey Sept. 16, 1958 2,914,637 Wuerth Nov. 24, 1959 FOREIGN PATENTS 565,302 Great Britain Nov. 6, 1944 

3. AN ELECTRICAL SWITCH FOR A STARTING-DEVICE FOR GASFILLED ELECTRIC DISCHARGE TUBES COMPRISING A PAIR OF NORMALLY OPEN CONTACTS, A FIRST BIMETALLIC ELEMENT HAVING A RELATIVELY FLAT PORTION ONE SIDE OF WHICH IS CONSTITUTED OF A METAL HAVING A HIGHER THERMAL COEFFICIENT OF EXPANSION THAN THE OTHER, SAID FIRST BIMETALLIC ELEMENT BEING ADAPTED UPON HEATING TO MOVE ONE OF SAID CONTACTS TO CLOSE THE PAIR, A SECOND BIMETALLIC ELEMENT HAVING A RELATIVELY FLAT PORTION ONE SIDE OF WHICH IS CONSTITUTED OF A METAL HAVING A HIGHER THERMAL COEFFICIENT OF EXPANSION THAN THE OTHER, SAID SIDE OF SAID SECOND BIMETALLIC ELEMENT HAVING THE HIGHER THERMAL COEFFICIENT OF EXPANSION ENGAGING THE SIDE OF THE FIRST BIMETALLIC ELEMENT HAVING A HIGHER THERMAL COEFFICIENT OF EXPANSION WHEREBY SAID CONTACT IS MOVED FIRST IN A DIRECTION TO CLOSE THE PAIR OF CONTACTS WHEN THE FIRST BIMETALLIC ELEMENT IS HEATED AND TO MOVE THE CONTACT IN AN OPPOSITE DIRECTION WHEN THE SECOND BIMETALLIC ELEMENT IS HEATED TO OPEN THE PAIR OF CONTACTS, A PLATE-SHAPED HEATING ELEMENT IS DIRECT PHYSICAL CONTACT WITH SAID FIRST BIMETALLIC ELEMENT, SAID HEATING ELEMENT BEING CONNECTED IN PARALLEL WITH SAID CONTACTS WHEREBY THE CLOSING OF SAID CONTACTS SHORT-CIRCUITS SAID HEATING ELEMENT, SAID HEATING ELEMENT BEING CONSTITUTED OF A MATERIAL THE RESISTANCE OF WHICH DECREASES WITH INCREASING VOLTAGE. 